Fusarium diseases

>> Fusarium wilt - Fusarium oxysporum
>> Foot and root rot - Fusarium solani
>> Bakanae disease of rice - Gibberella fujikuroi
>> Gibberella fujikuroi on maize
>> Gibberella zeae

Fusarium wilt - Fusarium oxysporum

Key characteristics

• Leaf chlorosis
• Stunted growth
• Vascular browning of stem and root tissue
• Bananas will wilt, the stems will crack and they will die in 1-2 months
• Tomato, cucurbits and other host plants will wilt temporarily during the hotter parts of the day for a few days before dying.

Introduction

Fusarium oxysporum Schlecht. Emend. Snyder & Hansen includes many representatives which collectively cause vascular wilt diseases of a wide range of crops including vegetable crops, ornamentals, bananas and palms. The isolates are usually host specific and many formae speciales (f. sp.) and races also occur in some f.sp. and these are specific to certain cultivars.

Disease Symptoms

Banana

The first internal symptom of Panama wilt is a reddish brown discolouration of the xylem tissue of the roots and rhizome. Subsequently, after colonisation of the pseudostem, the first aboveground symptoms occur. The inner parts of the leaf sheaths may have reddish brown spots. The oldest leaves become bright yellow and wilt, and leaf sheaths at the base of the stem show obvious cracking. The stem cracking is a key diagnostic symptoms of this disease. As the younger leaves progressively die, the xylem of the stem tissue becomes reddish brown and within 1-2 months, the stem decays until it collapses. These symptoms are typical for the 'chuoi tay' group in the northern part of Vietnam and the 'chuoi xu' in the south.

Tomato

The key symptom of Fusarium wilt of tomato is stunting, dark brown vascular discolouration, leaf chlorosis and wilting until death.

In seedling infection, the older leaves droop and curve downward, vascular tissue darkens, and the plant wilts and dies. In older diseased plants, the leaves yellow after blossoming. The plant then wilts during the hotter parts of the day and in a few days, collapses and dies. The vascular tissue in the stem is dark brown but the pith remains healthy. Fruit infection can occur displaying the same brown vascular discolouration. Fusarium wilt of tomato can be associated with infection by root knot nematode.

Cucurbits

There is general yellowing of the foliage at any growth stage with sporadic occurrence through the field. Vascular browning occurs in the taproot and stem, and wilting can occur quite suddenly, especially late in the season when plants have a high fruit load.

Description of the Pathogen

Under the microscope, F. oxysporum can be differentiated from other Fusarium species. On carnation leaf agar, the pathogen is identified by the production of oval microconidia in false heads arising from short monophialides. The macroconidia are produced in sporodochia, they are 3-5 septate and fusoid with a foot cell at one end. Chlamydospores are usually abundant, formed singly, in pairs or chains. On potato dextrose agar, the mycelium can be white to orange to pale violet with sporodochia often produced in the centre of the colony.

Distribution in Vietnam

This disease has been recorded in all parts of Vietnam.

Host Range

Host specific types of F. oxysporum cause wilt in many plant species. The key hosts in Vietnam include the following:

• banana F. oxysporum Schlechtend.: Fr. f. sp. cubense (EF Sm.) WC Snyder & HN Hans.
• tomato F. oxysporum Schlechtend.: Fr. f. sp. lycopersici (Sacc.) WC Snyder & HN Hans.
• cucurbits F. oxysporum Schlechtend.: Fr. f. sp. cucurbitacearum Gerlagh & Blok.

Epidemiology

F. oxysporum persists in soil for long periods as chlamydospores in plant residues until germination is stimulated by host roots, root excretions from non-host roots, or contact with pieces of fresh non-colonised plant remains.

Following infection of susceptible host roots, the fungus colonises the vascular tissue to cause disease. In the advanced stages of the disease, the fungus grows out of the xylem tissue into the adjacent parenchyma, producing extensive hyphae and chlamydospores. The chlamydospores are returned to the soil when the diseased plant residues decay. They can remain viable in the soil in their dormant state for several years, and renew upon germination and parasitic or saprophytic colonisation of a new host.

The saprophytic activity of F. oxysporum is not a means of survival, but rather a way to increase inoculum levels to a point where the host will succumb to disease. A relatively high population level, compared to other soil-borne pathogens is necessary for infection, especially in banana as multiple infection sites are required for aboveground symptoms to occur.

The optimum temperature for infection is 28°C, with the same moisture, nutrition and pH requirements that favour plant growth.

Dispersal is limited but chlamydospores can be transported by activities such as irrigation, cultivation and the movement of contaminated soil and propagating material. Infected young banana suckers are a source of contamination of new sites. In tomato, the stakes are a common dispersal mechanism of infected soil. Contaminated soil can also be carried on farm animals and people as well as ploughs and in irrigation water.

There is often an association with Fusarium wilt and nematode colonisation, where the nematodes provide a potential entry point for the fungus and also the bacterial wilt pathogen. This complex is more common in cucurbits and tomatoes than bananas. It is common to find root knot nematode galls on roots of tomato infected by Fusarium wilt.

Differentiating between wilt diseases

Note that bacterial wilt will produce ooze from lower stem tissue that exudes in water upon sectioning and there is less pronounced vascular browning. The distinction is important with respect to control as R. solanacearum does not produce a survival structure, so that its inoculum levels decline within a few months in the absence of a host. In contrast, F. oxysporum produces chlamydospores that survive for many years. Verticillium wilt has not yet been confirmed in Vietnam. It is a fungal disease which causes similar symptoms to Fusarium and bacterial wilts.

Control

The most vulnerable point in the disease cycle is the lack of a natural dispersal mechanism. Care must be taken with respect to exclusion and early quarantine, the best control measures. Resistant varieties should be grown where available. By raising the soil pH to 6.5-7 and using nitrates rather than ammonia for nitrogenous fertilisation, disease development is limited. Contaminated irrigation water shuold be avoided. Crop rotation is generally ineffective due to the effective survival strategies of the pathogen.

For the control of Panama wilt of bananas, do not use suckers from infected plants, even if they are not displaying any symptoms. Infected banana plants should be removed from the plantation to prevent further spread of the disease.

Symptoms of infection in pseudostem

Vascular discolouration of banana caused by F. oxysporum f. sp. cubense 
 

Yellowing of banana leaves caused by F. oxysporum f. sp. cubense

Stem cracking of banana caused by F. oxysporum f. sp. cubense 
 

Wilted banana plant infected by F. oxysporum f. sp. cubense

Fusarium oxysporum lycopersici symptoms on tomato

F. oxysporum macroconidia

F. oxysporum microconidia 
 

F. oxysporum in situ

F. oxysporum culture characteristics 
 

Foot and root rot - Fusarium solani 

Key characteristics

• Red streaks develop at the base of the plant.
• The older root tissue becomes necrotic.
• Adventitious roots develop near the soil surface to compensate for loss of lower roots.
• Yield is reduced as the plants are stunted.

Introduction

Fusarium solani (Mart.) Sacc. foot rot, red root or dry root rot most commonly occurs in beans. The disease causes minimal damage in unstressed plants, but is induced under conditions of drought or flooding. A red rot occurs at the base of the stem resulting in stunted growth and poor yields.

Disease Symptoms

Red rot or dry root rot causes a yellowing or reddening at the base of the stem and the drying out of the taproot. The lower root tissue may completely rot, and secondary roots may form above the diseased area. Stunting, wilting and fruit rot may also occur.

Symptoms include narrow, longitudinal, red to brown streaks on the roots of young plants. These areas become necrotic but are usually confined to the cortex. If the soil is not restrictive to root development, the cortical tissue may regenerate and the plant may recover. Otherwise, the root system is destroyed. Adventitious roots may appear near the soil surface, and it is these that the stunted plants in the crop rely on for survival. Primary leaves may yellow and drop and crop vigour is irregular within the field.

Description of the Pathogen

Under the microscope, F. solani can be differentiated from other Fusarium species. On carnation leaf agar, the pathogen is identified by the production of oval microconidia in false heads arising from very long monophialides. The macroconidia are produced abundantly in cream sporodochia, they are 3-5 septate and fusoid with a foot cell at one end. Chlamydospores are usually formed in 2 to 3 weeks. On potato dextrose agar, the mycelium can be white to cream with cream or bluish green sporodochia often produced in the centre of the colony.

Distribution within Vietnam

This disease has been recorded in all parts of Vietnam.

Host Range

The host range of F. solani includes bean, pumpkin, squash, marrow, coffee, lychee and soybean.

Epidemiology

Compacted soil and low soil temperatures, especially at planting favour disease development. Yield reductions are greater when water is limiting. The pathogen is soil borne and seed borne, but may also be dispersed locally by wind in infected plant debris. It survives in the soil as chlamydospores. Germination of the chlamydospores is stimulated by exudates from seedlings and root tips. This dictates the common association of F. solani with damping off and root rot diseases. The chlamydospores can germinate and reproduce in the absence of a susceptible host, improving the survival of the pathogen in the soil.

Control

• Wait until the soil temperature rises to 20°C before sowing seeds.
• Loosen the soil before planting or avoid compacted areas.
• Hill the soil at the plant base to encourage root growth above the diseased tissue.
• Maintain optimum moisture levels throughout the growing season.
• Select resistant cultivars.
• Ideally, to reduce the inoculum levels in the soil, avoid planting beans for five years and cucurbits for three years.

F. solani foot rot of bean

F. solani root rot on Aroids

F. solani macroconidia 
 

F. solani microconidia

F. solani culture characteristics 
 

Bakanae disease of rice - Gibberella fujikuroi

Introduction

The name ‘bakanae’ means bad or naughty seedling in Japanese, referring to the etiolation symptom caused by the disease as a result of gibberellin production of the pathogen upon infection of the host. The causal pathogen is Gibberella fujikuroi (Sawada) Ito in Ito & K. Kimura mating population (MP) C with the anamorph Fusarium fujikuroi.

Although bakanae disease usually causes death or sterility of rice, mycotoxin contamination is of concern as the pathogen is seed-borne. The extent of this contamination is important due to the potential risks involved.

Disease Symptoms

Symptoms vary according to the strain, inoculum levels and toxins present. High inoculum density results in seedling blight, stunting and chlorosis. Low levels result in etiolation symptoms due to gibberellin production. Strains that produce high levels of gibberellins cause seedlings to grow tall, thin and yellow and die. If high levels of fusaric acid are produced, infected seedlings are stunted, chlorotic, they develop crown and root rots and die. Next to healthy seedlings, diseased plants are very conspicuous. If older plants survive to maturity, they have tall, thin tillers, pale flag leaves, adventitious roots at the lower nodes and sterile panicles.

Description of the Pathogen

The asexual macroconidia of MP-C of the G. fujikuroi species complex (Fusarium fujikuroi) are 3-5 septate and curved towards the tips. The microconidia are produced in chains and false heads.

Geographical Distribution

Asia, America, African rice growing areas have this disease and it can be found mostly in the northern part of Vietnam.

Host Range

Rice Oryza sativa

Epidemiology

The disease causes more damage in the summer crop than the spring crop as disease expression is favoured by high temperatures. Transplanted rice plants display more symptoms than those grown from broadcast seed. Improved varieties are less susceptible to this disease than the traditional rice varieties.

Control

• Plant healthy seed.
• Remove diseased plants when weeding the crop.
• The fungicides Benomyl and Bavistin can be used, however they may only be effective if the soil around the base of each plant is dry.
• When transplanting, avoid damaging the root tissue to prevent a potential entry source for the pathogen.

Field symptoms with tall, yellow, infected plants (photo: John Leslie)

Diseased rice plants on the right are much taller (photo: John Leslie)

Gibberella fujikuroi on maize

Introduction

The occurrence of perithecia belonging to the G. fujikuroi MP-A on maize stubble in Northern Vietnam has been recorded. The implications of this discovery relate to potential mycotoxin contamination and the subsequent end-use of maize products. The economic losses for maize from this disease are not known. Mating population A does however produce the mycotoxin fumonisin. Fumonisins have been shown to cause equine leukaencephalomalacia andporcine pulmonary edema and are hepatocarcinogenic in rats. The World Health Organisation classes fumonisins as Class 2B carcinogens. This pathogen is responsible for maize root, stalk and cob rot.

Disease Symptoms

The symptoms are generally rotting of the roots, plant base and lower internodes. Rot normally begins soon after pollination and becomes more severe as the plant matures. A white-pink, salmon or purple discolouration of the pith, stalk breakage and premature ripening occurs. These symptoms are also characteristic of G. zeae infection, but this stalk rot has a red discolouration rather that the salmon. Superficial perithecia are also produced on the stalks.

Description of the Pathogen

The asexual macroconidia of MP-A of the G. fujikuroi species complex (Fusarium verticilloides) are 3-7 septate and curved towards the tips. The microconidia are produced in chains and false heads.

Distribution within Vietnam

The perithecia were discovered in Northern Vietnam.

Host Range

Zea mays, Sorghum bicolor and various grass species.

Epidemiology

Infection is favoured by a dry early season, with warm 28-30°C wet weather, 2-3 weeks after silking. This disease occurs mostly in summer-autumn crops in the Northern part of the country and the wet season in the southern parts.

Control

Select improved hybrids and resistant varieties to grow. It is thought that the stalks sampled were on old varieties of maize rather than the modern commercial hybrids. Balanced soil fertility, avoiding low potassium and high nitrogen also helps prevent disease. A lower planting density is also recommended. Avoid harvesting the corn during wet weather to prevent postharvest rots.

Perithecia of Gibberella fujikuroi on maize residue 
 

Asci of Gibberella fujikuroi

Asci and ascospores of Gibberella fujikuroi
 

Perithecia on corn husk   

Perithecia on corn husk   

Gibberella zeae

Introduction

Gibberella zeae (Schwein.) Petch is responsible for major losses in corn, largely due to stalk rots and cob rots. G. zeae infection can lead to mycotoxin contamination of grain. When female pigs consume the contaminated grain, their reproductive system is affected by the zearalenone content.

Disease Symptoms

The leaves of infected plants suddenly turn a dull greyish green, while the lower internodes soften and turn tan to dark brown. The stalks have pink to red discolouration on the internal diseased tissue. The fungus causes shredding of the pith and may produce small round black perithecia superficially on the stalks. Lesions may develop concentric rings.

Description of the Pathogen

The perithecia are bluish black and spherical. When mature, asci containing eight ascospores develop. Ascospores are 3-septate, slightly curved and tapering towards the ends. The asexual macroconidia of G. zeae (Fusarium graminearum Schwabe) are 3-5 septate curved and tapering towards the ends. Microconidia are not produced, some isolates produce chlamydospres and PDA pigmentation is red with pink mycelium.

Distribution within Vietnam

The pathogen causes disease in the northern part of the country and during the wet season in the southern parts.

Host Range and Epidemiology

Perithecia on infected maize stalks mature under warm, wet conditionsto exude mature ascospores. These are dispersed on the wind to ears or stalks where they germinate and penetrate healthy host tissue. Mycelium may develop on diseased plant parts during warm moist weather. The fungus may then overseason in infected debris and seed. Hosts to this pathogen include maize, wheat, barley, and other cereals, where it also causes scab and seedling blight. This disease occurs mostly in summer-autumn crops in the northern part of the country and the wet season in the southern parts.

Control

Select improved modern commercial hybrids and resistant varieties to grow rather than old varieties of maize. Balanced soil fertility, avoiding low potassium and high nitrogen also helps prevent disease. A lower planting density is also recommended. Avoid harvesting the corn during wet weather to prevent postharvest rots.

Corn shelling

Collecting corn stubble with perithecia

G. zeae  
 


G. zeae on corn 

G. zeae on corn 

G. zeae on CLA